What is the period in which one can become pregnant (with menstrural cycle of every 28 to 30 days) and when is it safe to do to prevent pregnancy ?

Pregnant any time: You can become pregnant at any time during your cycle, but the highest chance is during/around time of ovulation (egg-release). During a normal ovulatory cycle, the ovulation occurs 14-days before the menstrual flow starts. So, for 28-day cycle, ovulation day=14. For 30-day cycle, ovulation on day=16. To prevent unwanted pregnany, contraception is recommended at all time! consult doc. Good luck.